A conventional power supply apparatus includes an ASK modulator for carrying out ASK modulation (pulse modulation with a duty ratio of about two) of a radio frequency signal (RF signal), and for outputting a modulation signal that is a pulse signal; an amplifier for amplifying the pulse signal output from the ASK modulator; and an antenna for sending out the pulse signal amplified by the amplifier to noncontact wireless communication equipment.
In this case, the ASK modulator maintains the duty ratio of the pulse signal, and the amplifier amplifies the pulse signal in such a manner that the pulse signal transmitted from the antenna has constant average power.
Thus, the noncontact wireless communication equipment, receiving the pulse signal transmitted from the power supply apparatus, charges an internal capacitor with the pulse signal (see non-patent document 1, for example).
Non-patent document 1: Mituo Usami, “An Ultra Small RFID Chip: μ-chip”, MWE2003 Microwave Workshop Digest, pp. 235-238, Published by Central Research Laboratory Hitachi, Ltd., 2003.
With the foregoing configuration, the conventional power supply apparatus can charge the capacitor of the noncontact wireless communication equipment by transmitting the pulse signal as long as the distance to the noncontact wireless communication equipment is short enough such as several tens of centimeters. However, the average power or instantaneous power of the pulse signal transmitted from the antenna is not increased even if the distance to the noncontact wireless communication equipment lengthens. Thus, when the distance to the noncontact wireless communication equipment elongates, the conventional power supply apparatus has a problem of being unable to charge the capacitor of the noncontact wireless communication equipment even if the pulse signal is transmitted.
The present invention is implemented to solve the foregoing problem. Therefore it is an object of the present invention to provide a power supply apparatus capable of charging the capacitor of the noncontact wireless communication equipment even if the distance to the noncontact wireless communication equipment is long.